Microenvironmental heterogeneity and space utilization by desert vines within their host trees

The three-dimensional biomass distribution and the microenvironments experi enced by several desert Vine species growing within the canopy of host tree s were studied at the Centro Ecologico de Sonora in Mexico. The light envir onment within the crown of the host tree Cercidium microphyllum showed a ho rizontal and vertical gradient from the base of the trunk to the edges of t he canopy. Within this gradient total daily photosynthetically active radia tion (PAR) varied from 478 mol m(-2) outside the crown to 4.6 mol m(-2) at the centre of the crown and close to the ground. Maximum air temperature wa s 3 degrees C lower beneath the crown than outside. Within the canopy, most vines experienced less than 50 % of the daily available PAR outside the ca nopy. For most of the day, leaves of vines received 15 % or less of the max imum available PAR. Our study shows that vines do not grow towards full sun light but rather they exploit different habitat possibilities within their host tree crown. Leaves along the stems of vines experienced a wide range o f light environments, showing coefficients of variation (CV) in total daily PAR from 36.4 to 94.6 %. Daily courses of PAR also showed that leaves with in the canopy experienced short-term temporal variation in the light enviro nment. Differences in CV of daily PAR values and preferences in heterogeneo us light microenvironments among species suggested that different vine spec ies might be spatially separated in the canopy. We suggest that in desert h abitats, conditions within the crown of host trees result in an important m icrohabitat that vines can exploit, allowing them to avoid the high light, temperature and water deficits found in the surrounding environment. (C) 19 99 Annals of Botany Company.